Photographs, in recent years, are handled in the form of image data in increasingly more situations as people have come to enjoy widespread use of digital cameras, mobile phones with a built-in camera for a digital camera function, and similar devices. This trend has triggered the proliferation of televisions and like display devices which are capable of displaying photographs transferred from digital cameras, mobile phones, etc. and of printers and like image forming devices which are capable of printing the photographs.
Meanwhile, large volumes of data, such as image data, are more and more frequently transferred between information devices. The transfer of large volumes of data is very often carried out using wireless communication technology. Infrared communications are especially popular among various wireless communication technologies for its advantages listed below:                Unlike radio waves, there is no legal regulation governing the use of infrared frequencies. Global standards can be established.        Transmitters and receivers are compact, lightweight, and inexpensive. Their power consumption is low.        Undesirable information leakage and interference seldom occur because light travels in a straight line. For the same reason, space can be used efficiently.        
IrDA standards, established by an industry organization IrDA (Infrared Data Association), is an example set of infrared communications standards. IrDA standards divide into some layers of standards (a hardware layer, a data link layer, a protocol layer, etc.). IrDA standards are especially popular in data transfer between devices like mobile phones, mobile information terminals, laptop computers, digital cameras, printers, and electronic wrist watches over a distance of few meters within a PAN (Personal Area Network).
However, traditional IrDA standards require repeated bidirectional communications for procedures including search for a second party and its identification and negotiation, in order to confirm that both parties are ready for communications. The transfer rate during the repetition of the procedures is specified at a relatively low value, which we regard as a problem. In addition, the procedures inevitably entail some waiting periods which add to the total communications time. This is another problem.
Search for a second party is not always needed in infrared communications because the user often directs the light emitting section of the transmitting-end device at the light receiving section of the receiving-end device by himself/herself before a data transmission. Considering this fact, Japanese Unexamined Patent Publication (Tokukai) No. 2005-347886 (published Dec. 15, 2005; hereinafter, “patent document 1”), as an example, offers switching between IrDA communications and non-IrDA communications. The technology thus achieves high-speed data communications when there is no need to search for potential receiver apparatus in infrared communications. Specifically, the data transmitting device of patent document 1 comprises: first communications means for transmitting data according to IrDA standards; second communications means for transmitting data not according to IrDA standards; and switching means for switching between a process carried out by the first communications means and a process carried out by the second communications means. The second communications means transmits management information for managing data to be transmitted. The data transmitting device then receives, from a device responding to the transmitted management information, communications capability information indicating the communications capability of that device. The data to be transmitted is transmitted in packets based on the received communications capability information. With these procedures implemented, one can omit the search for potential receiver apparatus in infrared communications.
Patent document 1 is an example of proposed high-speed data communication technology viable when there is no need to search for potential receiver apparatus in infrared communications. Other examples include the communications standards called IrSimple and the communications methods based on half-duplex communications mode abbreviated as IrSimpleShot® and IrSS®. For more details of the IrSimple, see “Standardization of High-speed Infrared Communications Protocol IrSimple” by NAOE Hitoshi, et al. Sharp Technical Report, Vol. 95, February, 2007, page 63-68.
FIG. 16 shows transmission timing for a connection frame SNRM (SIR (9,600 bps)-RZI) and data frames UI (FIR (4 Mbps)-4 ppm) in conventional IrSS-based half-duplex communications. As shown in the figure, in IrSS-based infrared communications, an SNRM (Set Normal Response Mode) frame is transmitted prior to the transmission of data frames (UI (Unnumbered Information) frames) in FIR (fast infrared) mode. The SNRM frame is a connection frame (communications information messaging frame) in low-speed SIR (serial infrared) mode (9,600 bps). The “frame” in this context is identical in meaning to the term frame typically used in the field of communications and refers to a chunk of information with a defined beginning and end. The “connection frame” is a collection of information needed to establish a communicable link between a transmission-end device and a receiving-end device.
Discussion is in progress also on communications using visible light which has similar properties to infrared radiation. Applications to half-duplex communications like IrSS are especially expected.
However, with the conventional technology above, the receiving-end device is sensitive to infrared noise produced by, for example, a liquid crystal television containing a cold cathode tube, a plasma television, or an inverter-type fluorescence lamp. The device may not be able to receive the connection frame normally, hence possibly failing to properly receive data frames which are transmitted following the connection frame.
FIG. 17 shows a signal pattern in SIR mode. As shown in the figure, the data bits 0 and 1 in the RZI (Return to Zero Inversion) modulation used in SIR mode are represented by the presence and absence of a pulse during a predetermined period. For example, in the SIR mode (9,600 bps) used for the transmission of a connection frame, a pulse, 1.41 μsec to 22.13 μsec wide, is inserted in a period of about 104 μsec for a data bit 0, whilst no pulse is inserted in a period of about 104 μsec for a data bit 1.
The connection frame that should be received in SIR mode may be buried in noise in an environment where there are a lot of infrared noise pulses, about a few microsecond to a dozen microsecond wide: for example, when there is an inverter-type fluorescence lamp near the receiving device. The noise could be erroneously recognized as pulses representing data bits and disrupts normal reception of the connection frame. As a result, the receiving-end device fails to switch to FIR mode for data frame reception and cannot receive data frames transmitted in FIR mode.
Also, it is known that IrSS-based infrared communications can be disrupted when the receiving device is a liquid crystal television containing a cold cathode tube, a plasma television, or a like device which emits infrared noise. The radiant infrared may reflect off a human body or a physical object and produce infrared noise pulses, about a few microsecond to a dozen microsecond wide, disrupting the communications similarly to the previous case. The adverse effect of infrared noise produced by the reflection off a human body is especially obtrusive in IrSS-based infrared communications because the user holds a transmission-end terminal and approaches the infrared receiving section of a receiving-end device up to a distance of about 20 cm to 1 m, for manipulation. Infrared noise is also known to increase when the temperature of the cold cathode tube falls below about 0° C. to 10° C. For example, if the liquid crystal television is powered on at low temperature, in some cases, it takes about 10 minutes to raise the temperature of the cold cathode tube and reduce the effect of infrared noise.
The infrared noise again seriously affect the reception of the connection frame if the receiving circuit in the receiving-end device contains AGC (Auto Gain Control). AGC functions to reduce gain for a strong reception signal and increase gain for a weak reception signal, in order to maintain the amplified signal level at a constant value. The infrared noise, as mentioned above, is a result of the reflection off a human body or a physical object. Since the reception signal level is generally weak, the AGC works to increase gain. Therefore, the AGC operates to increase the gain, for example, during the non-pulse period representing a 0 during the reception of an SIR-mode connection frame (9,600 bps). The infrared noise, otherwise buried in the connection frame, could be erroneously recognized as a reception signal (pulses representing data bits).
The non-pulse period during frame transmission is longer in SIR mode (9,600 bps) than, for example, in SIR mode (115.2 kbps) and FIR mode (4 Mbps). In the receiving-end device, the AGC operates to increase gain for infrared noise, which adds to the chances of erroneously recognizing the infrared noise as a reception signal.
Infrared noise could hamper data reception by the data transmitting device of patent document 1 as in the previous cases when it is used in half-duplex infrared communications in which the management packet (connection frame) and the image data (data frames) are transmitted by different signal formats. More specifically, If the signal format used for the transmission of the connection frame is more sensitive to infrared noise than the signal format used for the transmission of the data frames as in IrSS-based infrared communications, the connection frames may not normally be received due to the infrared noise, making it difficult to receive the data frames.
Future infrared communication technologies will likely involve specifications which develop from IrDA standards, like IrSS, and would have the same nature as IrSS. In addition, given the same modulation scheme, noise resistance can still be improved by introducing error recovery by error correcting code, for example, Reed-Solomon code. However, it is not easy to change the signal format for the connection frame that is transmitted before any other frame, while securing compatibility with traditional standards. It is unlikely that the signal format for the connection frames will ever be changed.
The description above is concerned with problems of conventional data transmitting devices which transmit a connection frame and data frames by infrared communications. Similar problems occur with data transmitting devices which transmit a connection frame containing transmission conditions for the data frames using a non-infrared transmission medium before the data frames are transmitted.